This invention relates generally to angular rate sensors of the vibrating element type, and particularly to a north-seeking gyroscope incorporating vibrating drive and sensing transducers in a paired tuning fork configuration mounted to rotate about a rotational axis oriented perpendicular to the sensing axis of the transducers, and a two axis low bias angular rate sensor.
The use of piezoelectric ceramic crystals in a paired tuning fork configuration for angular rate sensing is well known to the art, with representative examples being shown in U.S. Pat. Nos. 4,628,734 to Watson and 4,671,112 to Kimura. In each of these systems a pair of drive elements (or transducers) are energized to induce flexure and controlled vibration of those drive elements, with that vibration being transferred to a pair of parallel and closely-spaced sensing elements which extend longitudinally from the drive elements and vibrate in opposition about a nodal axis. Spatial movement of the vibrating sensor elements induces a sensed output signal that may be monitored and filtered to characterize the angular rate of the sensing elements and therefore the physical object to which the sensing elements are mounted.
U.S. Pat. No. 2,716,893 to Birdsall represents one of the initial disclosures of the use of piezoelectric crystal elements mounted on a rotor to sense acceleration signals which may be modulated by rotation of that rotor to derive an angular rate. The two elements in Birdsall '893 are not energized to induce flexure and vibration, but instead depend upon momentary physical displacement of the elements from the instantaneous plane of rotation of the elements about the axis of rotation of the rotor to induce stress forces on the elements caused by angular rates and accelerations. The magnitude of these stress forces are proportional to both the angular velocity (spin rate) of the rotor and the separation distance between the two elements perpendicular to the rotational axis of the rotor. While the scale factor and sensitivity of the Birdsall '893 system increase with the spin rate of the rotor and separation distance between the elements, its susceptibility to external vibrations and noise similarly increases. Moreover, the Birdsall '893 system does not measure angular rate per se, but rather requires derivation of an angular rate from measurement of acceleration forces.
U.S. Pat. No. 4,444,053 to Rider discloses an inertial sensor for aviation attitude and heading reference systems utilizing four transducer elements on a rotor to measure angular velocity and linear acceleration. The Rider '053 inertial sensor is effectively an operable embodiment of the system described in Birdsall '893 made feasible and competitive with conventional gyroscopes by improvements in circuitry and sensor element construction achieved since the time of Birdsall '893. Because the available bandwidth depends upon the rotor spin rate, the Rider '053 system must preferably be operated at its highest practical spin rate of 52 Hz (3,120 rpm), which is between one fourth and one eighth the normal spin rate of a conventional gyro. The system also weighs nearly 7 kg (15 lbs) and constitutes a significant expenditure.
North-seeking gyroscopes or gyrocompasses are traditionally characterized by reference to a freely rotating gyroscope rotor having damped precession about its own axis of rotation which naturally aligns parallel to the earth's axis of rotation (spin axis) and perpendicular to the local centripetal acceleration vector due to the effective torque induced by the horizontal component of the coriolis force. However, because the rate of change of the gyroscope's angular momentum vector equals the applied torque, a gyroscope having a large angular momentum vector influenced by a comparatively small torque (i.e., proportional to the earth's coriolis force) will require significant time to align the angular momentum vector with the axis of rotation producing the torque. Since the period of precession is generally independent of the angular separation between the earth's and the gyroscope's rotational axes, and the angular momentum vector is limited to an operating minimum by the spin rate and physical properties of the rotor, the time necessary to achieve alignment within a specified degree of accuracy will generally be significant and will increase dramatically as accuracy narrows.
U.S. Pat. No. 3,987,555 to Haagens discloses a north-seeking gyroscope utilizing capacitance sensing elements to measure torsional displacement of the rotor about a specialized hinge which provides two degrees of freedom for the rotor. The Haagens '555 system thereby uses the effect of the earth's spin on the sensing elements to detect the orientation of the earth's spin axis. However, the Haagens '555 system requires synchronizing the charging half-cycles of the capacitance sensing elements with the orientation of the rotor, and preferably utilizes a closed-loop servo system to null the output signal of the sensing elements by reorienting the axis of rotation of the rotor at 45.degree. relative to the earth's axis of rotation. The Haagens '555 system can alternately rely upon splitting the output signal into two channels representing orthogonal sectors, and vectoring the signals to resolve geographic north. However, this also requires a more complex modulation control and signal filtering system, and would operate more slowly than the closed-loop servo embodiment.
U.S. Pat. No. 3,938,256 to Crocker discloses a two-degree-of-freedom north-seeking gyrocompass utilizing a servo system to null the output signal that is described as quick settling and being capable of providing an accurate short-reaction-time heading reference. The Crocker '256 system describes using a four gimbal assembly and circuits for compensating erection error signals to reduce the settling time for a conventional north-seeking gyroscope from several days to approximately 30 minutes (.+-.10%) for acceptable operating accuracies.
Various other improvements in the mounting platforms, rotor configurations, and filter circuitry of north-seeking gyroscopes and heading reference systems are known to the art, with representative examples being shown in U.S. Pat. Nos. 3,750,300 to Tumback; 4,379,365 to Riethmuller; 4,442,723 to Auer; 4,443,952 to Schulien; 4,512,086 and 4,599,803 to Galuschak; 4,530,164 to Barriac; 4,461,089, 4,075,764 and 4,321,678 to Krogmann; 4,603,483 to Wing; 4,622,646 to Waller; 4,686,771 to Beveventano; 4,791,727 to Hojo; and 4,811,233 to Lauro. The accuracy and settling times for these systems vary significantly, as do their complexity, bulk, and cost. The gyrocompass disclosed in Galuschak '086 requires a settling time of approximately 6 minutes for maximum accuracy, which represents an improvement over Crocker '256, while Riethmuller '365 provides a rough estimate of geographic north with an accuracy of .+-.1.0.degree. in less than one minute (including initial vertical alignment of the pendulous bodies, rotor start up, zeroing time, and two rough estimates taken aling orthogonal vectors) and two fine measurements requiring slightly greater time for greater accuracy.